1. Field of the Invention
The present invention relates to a color signal conversion method and color signal converter for color image printing.
2. Description of the Related Art
In the field of color image printing and color hard copy, color signal conversion from the three primary colors of light (RGB) to the three primary colors of color (CMY) has been indispensable when reproducing color by a printer.
In regard to the actual ink, however, there has been the problem that color conversion cannot be carried out simply because other colors are mixed in the ink. A three-dimensional interpolation using various color conversion tables has been proposed as a way of quickly and easily converting color signals.
The three-dimensional interpolation is a method of carrying out color conversion in color space in which three-dimensional color space is divided equally into n units in each direction of RGB so as to subdivide one RGB color space into nxc3x97nxc3x97n units. By subdividing color space in this way, color can be handled as a linear form in color space, thereby shortening the amount of time necessary for calculation compared with the case in which color does not change linearly and simplifying the calculation itself.
As shown in FIGS. 1a-1d, each interpolation calculation is carried out in 4-point, 5-point, 6-point,and 8-point interpolations using data of four to eight apices, respectively.
FIGS. 1a-1d each show only one unit of a cube that has been cut into a number of units in each interpolation, each interpolation actually cutting a cube into four to eight unit cubes.
Three-dimensional interpolation calculation thus finds the current pixel data through linear interpolation using the four to eight points of apex data of the subdivided color space that contains the points currently being sought.
These interpolations divide three-dimensional color space represented by three primary colors into a plurality of color cube units, and within the color cube units, linear interpolation is carried out with color assumed to change linearly, thereby enabling fast and easy conversion of color signals.
A color signal converter of the prior art is disclosed in Japanese Patent Laid-open No. 99587/95. As shown in FIG. 2a, this converter comprises picture element input section 100, comparator 102, adder 101, addrress generator 103, color conversion table memory 105, selector 107, weitht generator 103, oblique triangular column determination section 104, and oblique triangular column interpolation calculator 109.
Picture element input section 100 inputs and divides RGB color image signals into higher-order bits(RH,GH,BH) and lower-order bits(RL,GL,BL). Comparator 102 compares the lower-order bits with themselves and outputs the compared results. Adder 101 adds the output of comparator 102 to that of picture element section 100. Address generator 103 generates from the outputs of both picture element input section 100 and comparator 102. Color conversion table memory 105 stores color conversion data for converting RGB to CMYK. Selector 107 selects the output of the color conversion table memory 105. Weight generator 108 generates, from the lower-order bits, weight coefficients for interpolation points. Oblique triangular column determination section 104 determines the size of the weight coefficients and selects oblique triangular column made up of apices of each unit cubes which are obtained by dividing three primary color spaces into a plurality of unit cubes. Oblique triangular column interpolation calculator 109 interpolates output values read from color conversion table memory 105 using the weight coefficients.
In this way, the color signal converter of the prior art effects color conversion by linear interpolation in a state free of ripple (a phenomenon in which false contours are generated by interpolation) in all color space.
FIG. 2b is a simplified block diagram of FIG. 2a. 
As shown in FIG. 2b, when RGB signals are received in pixel input section 100, the section 100 separates an RGBH signal for selecting color conversion table memory 105 and an RGBL signal for controlling oblique triangular column interpolation calculator 109. Address generator 103 generates from the RGBH signals memory address signals for selecting a memory address in color conversion table memory 105. Color conversion data that are specified by the addresses of the memory address signals are outputted from color conversion table memory 105. Upon receipt of color conversion data signals and RGBL signals, oblique triangular column interpolation calculator 109 carries out an interpolation calculation of color conversion data signals and outputs interpolation calculation data signals, which are the calculation results of interpolating the color conversion data signals.
Another color signal converter of the prior art is disclosed in Japanese Patent Laid-open No. 288706/95. As shown in FIG. 3, this converter comprises first color reproduction area table 150, second color reproduction area table 124, color reproduction area conversion section 121, entire corresponding color decision section 122, optimum color reproduction area selecting section 123, target color reproduction area by k-ink amount table 126, color area reproduction area dividing section by k-ink amount 125, entire corresponding ink amount storage memory 127, candidate selection section 128, interpolation calculation section 129. First color reproduction area table 120 stores RGB values. Second color reproduction area table 124 stores CMYK values. Color reproduction area conversion section 121 performs conversion of source representative color signals (RGB values) while varying the RGB values outputted from first color reproduction area table 120. Entire corresponding color decision section 122 searches target color reproduction area by K-ink amount table 126 and finds a set of CMYK ink amounts corresponding to the converted color signals. Optimum color reproduction area conversion selecting section 123 finds the optimum values of RGB values in color reproduction area conversion section 121. Target color reproduction area by K-ink amount table 126 stores data obtained by colorimetring color samples of a plurality of colors obtained by adjusting CMY ink amounts for a plurality of different K-ink amounts. Color reproduction area dividing section by k-ink amount 125 divides color reproduction area table by K-ink amount 126 into independent color reproduction area tables for each K-ink amount. Entire corresponding ink amount storage memory 127 stores one or more sets of CMYK ink amounts obtained by the optimum conversion RGB values. Candidate selecting section 128 selects one or two sets of CMYK ink amounts in accordance with K-ink amount adjustment coefficient. Interpolation calculation section 129 carries out interpolation calculation with K-ink amount adjustment coefficients as an internal division ratio to calculate CMYK ink amounts.
Thus, in the color signal converter of the prior art, CMYK ink amounts corresponding to any color of a color image display device are determined based on actual measurement data; and K-ink amounts are selected based on K-ink amount adjustment coefficients. Moreover, the CMYK ink amounts are selected with K-ink amount adjustment coefficients, and the CMYK ink amount in each K-ink amount is determined.
In the converter disclosed in Japanese Patent Laid-open No. 99587/95 has the following problems. These color conversion data are produced under a certain condition, so that color conversion data corresponding to the external temperature conditions are compelled to be rewritten to color conversion table memory 105 in the event of a change in the external temperature.
Furthermore, because especially in an ink-jet printer, ink spray conditions change according to the temperature of the ink or the amount of time the printer is used, and hence color reproducibility is decreased, color conversion data that correspond to current conditions are forced to be rewritten to color conversion table memory 105.
Moreover, large variations in color may occur after rewriting, resulting in variations in color reproducibility.
The converter disclosed in Japanese Patent Laid-open No. 288706/95 has the following problems. The converter uses a plurality of different tables of K-ink amounts for color conversion, finds the optimum ink amounts from target color reproduction area table by K-ink amount 126, and performs color conversion by interpolation calculation. To perform color conversion with this method, however, the color signal converter needs a large number of target color reproduction area by K-ink amount tables 126, and thus entails increased circuit scale.
In addition, changes in external condition necessitate either rewriting to target color reproduction area by K-ink amount table 126 or preparing in advance tables for every external temperature.
It is an object of the present invention to provide a color signal conversion method and color signal converter that can perform optimum color conversion with the smallest possible construction without decreasing color reproducibility despite changes in external conditions.
According to one aspect of the present invention, a color signal converter comprises first and second conversion tables and a color conversion interpolation section.
In a preferred embodiment, the first and second color conversion tables stores color conversion data for converting from RGB, which are the three primary colors of light, to CMY, which are three primary colors of color. The color conversion table interpolation section generates, based on external conditions and color conversion data outputted from the first and second color conversion tables, color conversion data signals for image generation.
In the event of change in the external conditions, a color conversion table that corresponds to the change in external conditions is generated by interpolating two color conversion tables for different conditions in accordance with the current external conditions. As a result, optimum color conversion can always be achieved without affecting color reproducibility by external conditions.
In addition, color interpolation according to the current external conditions realizes good color reproduction.
Moreover, the use of only two color conversion tables enables the capacity of memory for color conversion table to be decreased to a minimum. To attain good color reproducibility according to external conditions, a large number of color conversion tables must be provided for different external conditions and then switched or rewritten data in color conversion tables as necessary. With this invention, however, good color reproduction can be achieved by using a minimum of, i.e., only two color conversion tables for different external conditions.
No color shifts occur because the two color conversion tables are not switched or rewritten based on external conditions. Whereas large shifts in color may occur when switching or rewriting color conversion data in color conversion tables to guarantee good color reproducibility. Color shifts do not occur in this embodiment as changes in external conditions are adjusted by interpolation.
According to another aspect of the present invention, a color signal converter comprises a first color conversion table, a second color conversion table, a third color conversion table, a color conversion data selector, and a color conversion table data interpolation section.
In a preferred embodiment, the first color conversion table stores color conversion data for low external conditions. The second color conversion table stores color conversion data for high external conditions. The third color conversion table stores color conversion data for an intermediate condition between high and low external conditions. The color conversion data selector selects, according to external conditions, two color conversion data signals from three color conversion data signals outputted from the first to third color conversion tables, and outputs the selected color conversion data signals. The first and third color conversion tables are selected if the external condition is between the first temperature stored in the first color conversion table and the second temperature stored in the third color conversion table, and the second and third color conversion tables are selected if the external condition is between the third temperature stored in the second color conversion table and the second temperature stored in the third color conversion table. The color conversion table data interpolation section generates, based on external conditions and the two color conversion data signals outputted from the color conversion data selector, color conversion data signals for image generation that correspond to external conditions.
In the event of change in the external conditions a color conversion table is generated that corresponds to the change in external conditions by interpolating data in the three color conversion tables for different conditions in accordance with the current external conditions. As a result, optimum color conversion can always be achieved without affecting color reproducibility by external conditions.
Further, the use of only three color conversion tables enables the capacity of memory for color conversion table to be decreased to a minimum. To attain good color reproducibility according to the external conditions, a large number of color conversion tables must be provided for different external conditions and then switched or rewritten data in color conversion table as necessary. The present invention, however, can achieve good color reproduction by using a minimum of, i.e., only three color conversion tables for different external conditions.
Finally, no color shifts occur because the three color conversion tables are not switched and rewritten based on external conditions. While large color shifts may occur when switching or rewriting color conversion tables to guarantee good color reproducibility, color shifts do not occur in this embodiment as changes in external conditions are adjusted by interpolation.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.